The FeMo cofactor in the MoFe protein is believed to be the substrate ( e.g. atmospheric N 2) binding and reducing site in the nitrogenase system. The FeMo cluster, obtained by a different preparation method, is a subset of the FeMo cofactor. The cluster exhibits a very similar S=3/2 EPR signal, and inhibits the insertion of the cofactor back into the cofactor-deficient MoFe protein. However, the cluster does not contain homocitrate, and has one less Fe atom in the first coordination shell and has a change in the long range order. The cluster cannot activate the FeMo-cofactor-deficient form of the MoFe protein. A previous SAXS study shows that the FeMo cofactor solution is not homogenous; the cofactor exists in large aggregates (upto 25 w in R g) as well as in small oligomeric forms with an average R g of 6.96 w. We are interested in comparing the aggregation state of the cluster with that of the cofactor by using solution small-angle x-ray scattering.